Sealing a fiber drawing furnace

ABSTRACT

A drawing furnace or the like for manufacturing optical fiber preferably of a vertical fiber preform, which drawing furnace includes at least a preferably tubular heating element surrounding the fiber preform for heating the fiber preform, an outside insulating layer of the heating element, a frame part of the drawing furnace in order to place the heating element and insulating layer in the drawing furnace, and a cover part in order to close the area between the heating element and the frame part to insulate the insulating layer from the gas flow surrounding the fiber preform. A gas tube is fitted between the cover part of the drawing furnace and the heating element in order to feed gas to the gas space surrounding the fiber preform, and the cover part is still fitted mobile in relation to the frame when the length of the heating element changes in such a manner, that a force, which is substantially constant and seals the gas tube to the end of the heating element, is directed at the gas tube in all modes of operation of the drawing furnace.

The invention relates to a drawing furnace for manufacturing opticalfiber according to the preamble of the appended claim 1.

The manufacture of optical fiber can be roughly divided into twodifferent phases. A fiber preform is manufactured in the first phase.The fiber preform is a relatively short, rigid and thick quartz glassbar, to which the optical properties of the fiber being manufactured,i.e. a refractive exponent distribution parallel to the radius of thefiber, are formed in connection with the manufacture of said preform. Inthe next phase the preform is placed in a so-called drawing furnace,wherein the fiber preform is drawn to a thin and flexible optical fiber.In connection with the drawing of fiber, the fiber is also typicallycoated with one or more buffer coatings before it is reeled up forstorage.

In a drawing furnace, which is in a vertical position, the lower end ofthe fiber preform, which is placed vertically inside the furnace, istypically heated to a temperature of approximately 2000° C. with aheating element, which surrounds the fiber preform symmetrically in thehorizontal plane. This heating element is typically made of graphitematerial, the heating of which is arranged to take place inductively.When the lower end of the fiber preform reaches a temperature which ishigh enough, gravity draws the fused quartz glass, which is partly inquick condition, downwards, in which case this fiber is thus formed as acontinuous process. The diameter of the optical fiber that is formed inthe manner described above can be controlled e.g. by adjusting theheating of the fiber preform and by affecting the winding speed, i.e.the drawing speed.

In the manufacture of optical fiber with a drawing furnace, impurities,which drift to the fiber as fine particles, form a significant andwell-known problem as such. These particles may originate either fromoutside or inside the drawing furnace. Typically the impuritiesoriginating from the inside of the drawing furnace loosen from theheating element, insulating material or other high temperature parts ofthe furnace. The particles are typically, for example, silicon carbideor carbon. The glass material loosening from the fiber preform itselfmay form particles, which disturb the process. When meeting the fusedquartz glass forming into fiber, the impurity particles mentioned abovecause local faults (discontinuity) in the fiber, which affect thestrength of the fiber and may at a later stage cause, for example, thefiber to break.

For example, patent publication U.S. Pat. No. 4,547,644 discloses theseabovementioned problems caused by impurity particles in fiber drawing.As a solution to the problem, said publication presents sealing means,which have been arranged to the upper and lower ends of the heatingelement in connection with the insulating layer surrounding the tubularheating element of the drawing furnace, and which are used to decreasethe flow of impurity particles to the inside of the furnace. U.S. Pat.No. 4,547,644 presents a solution, which is substantially based on thecircular cover structure placed in the upper part of the drawingfurnace, which structure keeps, with its own weight, the upper one ofthe abovementioned sealing means at its place and allows thermalexpansion of the insulating material. The sealing reached with thesolution is, however, not completely satisfactory, because the sealingof the cover part in relation to the heating element of the drawingfurnace and other parts of the furnace is allowed to vary. The solutionpresented in U.S. Pat. No. 4,547,644 does not enable a precise controlof gas flow inside the drawing furnace either.

Flow conditions of the gases inside the drawing furnace are an importantfactor affecting the quality of the fiber being manufactured. Typicallythe aim is to arrange the gas flow from up to down, in which case thespeed difference between the gas flow and the fiber being formed is assmall as possible and no turbulence is formed between them. There is asolution known from background art, wherein a gas flow upwards insidethe drawing furnace is used, in which case the particles formed insidethe furnace are directed upwards with said gas flow, and thus do notmeet the downwards falling fused quartz glass, which is forming intofiber.

The abovementioned solutions of background art have not, however, beenable to solve the problems connected with the impurity particles in atotally satisfactory manner. Regardless of the good quality materialsused in the heating elements and in other components of the drawingfurnace and the special structure solutions developed for sealing thestructures of the drawing furnace, some particles, which disturb thedrawing of fiber, are still always formed inside the furnace. Using agas flow directed upwards causes problems in drawing fiber, when thedrawing speeds are great, for example over 600 m/s.

The main purpose of the present invention is to present equipment thatis used in the manufacture of optical fiber, with which equipment thesealing of the heating part of the furnace is improved and thus thetravel of impurity particles to the inside of the drawing furnace andthe fiber being formed in the furnace is prevented. In addition, withthe invention, control of the flow of gases fed to the drawing furnaceis significantly improved, as a result of which the gas flow surroundingthe fiber being formed behaves well from the point of view of the fiberforming process.

To attain these purposes, the drawing furnace according to the inventionis primarily characterized in what will be presented in thecharacterizing part of the independent claim 1.

The other, dependent claims will present some preferred embodiments ofthe invention.

The basic idea of the invention is that when optical fiber ismanufactured with a vertical drawing furnace, wherein the fiber preformis heated in the heating zone in such a manner, that thin fiber isformed of the molten being formed at the lower end of the fiber preformwith gravity and/or downward drawing aimed at the fiber, the cover partof said drawing furnace is fitted to the structure of the equipment insuch a manner, that thermal expansion of the heating element of thedrawing furnace cannot change the sealing between the cover part and theheating element. The cover part mentioned in an advantageous embodimentof the invention is fitted mobile in relation to the frame part of thedrawing furnace in such a manner, that the location of the cover part inrelation to the upper edge of the tubular heating element remainsconstant when the length of the heating element changes as a result oftemperature changes. Typically the length of the heating element of thedrawing furnace increases approximately 5 mm when the temperature of theelement increases from room temperature to a temperature ofapproximately 2000° C.

Preferably said cover part is attached to the frame part of the drawingfurnace with means, which press the cover part towards the heatingelement substantially with a constant force, regardless of the length ofthe heating element, in which case a constant force is directed also atthe sealing between the cover part and the heating element, i.e. theinner tubes of the gas tube, and the sealing remains constant.Preferably the pressing force of said means can be adjusted. In anadvantageous embodiment of the invention, the means according to theinvention are implemented with spring-like structures placed outside thehot space of the drawing furnace.

With the sealing structure according to the invention it is possible toeliminate the indefiniteness in the sealing between the drawing furnaceand its surroundings caused by thermal expansion, especially the thermalexpansion of the heating element, still, however, allowing the thermalexpansion of the heating element. Another significant advantage of theinvention is that the gas flow channels of the drawing furnace and thusthe gas flows themselves remain unchanged throughout the entiretemperature area, in which case the gas can be fitted to flow in thedrawing furnace at an optimal speed from the point of view of fiberformation, preferably at the drawing speed of the fiber being formed.

The following more detailed description of the invention, with examples,will more clearly illustrate, for anyone skilled in the art, preferredembodiments of the invention as well as advantages to be achieved withthe invention in relation to background art.

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows in a side-view, in principle, an embodiment of theinvention when the heating element of the drawing furnace is cold, and

FIG. 2 shows in a side-view, in principle, the embodiment according toFIG. 1 when the heating element of the drawing furnace is hot.

The appended figures are principled and, for better clarity, detailsirrelevant to understanding the invention have been left out, such asthe cooling channels in the upper part of frame part 11 of drawingfurnace 10.

FIG. 1 shows, in principle, the structure of the drawing furnace 10 usedin manufacturing optical fiber 3 and the location of different parts ofthe drawing furnace in relation to each other when the drawing furnaceis cold.

A quartz glass fiber preform 1, which is typically 50-100 mm in diameterand 500-2,000 mm in length, is placed vertically inside a tubularheating element 12. The material of the heating element 12 is typicallygraphite, which endures the temperatures of the process, which can bearound 2000° C., well. The heating element 12 is surrounded by aninsulating layer 13, which can be, for example, porous graphite wool.

Induction coil 14 is arranged to heat the heating element 12 withelectric induction, in which case a heating zone is formed against saidcoil, to which heating zone the lower end of fiber preform 1 is placed.Because of the heating effect of heating element 12, molten 2 is formedat the lower end of the fiber preform 1, which molten further forms to athin, downwards moving fiber with gravity and/or drawing directed atfiber 3.

In the drawing direction of fiber 3 after the heating element 12, aso-called extension tube 15 is arranged to the drawing furnace 10, thepurpose of which tube is to prevent the fiber from cooling too quickly.In order to manufacture good quality optical fiber 3 it is important,that the formed fiber is not subjected to a so-called cold shock. Here,cold shock means that stress states caused by too fast cooling remain inthe crystal structure of the fiber 3 being formed, in which case thefinal crystal structure of the fiber remains imperfect. This weakens theoptical properties of fiber 3 significantly, and also affects themechanical strength of the fiber.

The upper part of the extension tube 15 is advantageously silica tubetypically for the 100-500 mm long beginning part. The purpose of thispart is to separate the heating element 12 in a high temperature fromthe lower parts of the drawing furnace 10 (extension tube 15). A silicatube is suited for this purpose because of its high heat resistance, inwhich case the silica tube itself does not need to be actively cooled,and fiber 3 is thus not subjected to cold shock. Instead of quartz, forexample graphite can also be used as material for the extension tube 15.The formation of fiber 3 from fused quartz glass is still ongoing at thesilica tube, so therefore a too steep temperature gradient when movingdownwards from the area of influence of the heating element 12 would beharmful from the point of view of fiber formation by causing anon-desired cold shock to the fiber.

The downward flow 4 of inert gas, preferably argon gas, isadvantageously arranged to the drawing furnace 10. Said gas flow 4advances typically from the area of influence of the heating element 12further to the extension tube 15.

In order to create the abovementioned gas flow 4, a gas tube 16 isarranged to the upper part of the drawing furnace 10 according to theinvention. Gas tube 16 is formed of an outer tube 17 a and inner tubes18 a, 18 b arranged inside the outer tube. It is advantageous to placethe outer tube 17 a and inner tubes 18 a, 18 b concentrically around thevertical axis 10 x of the drawing furnace 10. The gas required for thegas flow 4 of the drawing furnace is fed from outside the furnace to achannel 17 b formed on the outer tube 17 a. Channel 17 b is preferablyplaced to go around the vertical axis 10 x of the drawing furnace 10horizontally. The vertical axis 10×of the drawing furnace 10 refers tothat vertical line of the drawing furnace, on which are the central linein height direction of the drawing space of the drawing furnace, thevertical line of the fiber preform 1, and the vertical line of the fiber3 being formed.

The upper inner tube 18 a and the lower inner tube 18 b are surroundedby the outer tube 17 of the gas tube 16. The purpose of inner tubes 18a, 18 b is to seal the space between the heating element 12 and thecover part 20 in such a manner, that the insulating layer 13 surroundingthe heating element is not in connection with the inside of the furnace,in which state fiber 3 is formed from the fiber preform 1. In a waycharacteristic to the invention, a hole 18 c is formed between the upperinner tube 18 a and the lower inner tube 18 b as a result of the designof the corresponding surfaces of the inner tubes. In an advantageousembodiment it is possible to form protrusions on the correspondingsurfaces of the upper inner tube 18 a and the lower inner tube 18 b,which protrusions prevent contact between the corresponding surfaces onthe entire surface area and, at the same time, the protrusions keep thedistance between the tubes constant, in which case also the size of thehole 18 c remains constant. It is very advantageous to design the hole18 c between inner tubes 18 a, 18 b such, that its opening towards thevertical axis 10 x of the drawing furnace 10 goes around the verticalaxis of the drawing furnace horizontally as a uniform circle. It is alsopossible to form the inner tubes 18 a, 18 b of the gas tube 16 from oneuniform tube, in which case the gas hole 18 c is formed in the saidtube, for example, by machining. In addition, it is possible to form theinner tubes 18 a, 18 b of the gas tube 16 from more than two parts, inwhich case, for example in order to form gas hole 18 c, separateintermediate parts can be used.

Channel 17 b of the outer tube 17 a of the gas tube 16 and the hole 18 cbetween the inner tubes 18 a, 18 b are connected to each other, in whichcase gas can flow from the hole between the inner tubes through thechannel of the outer tube to the inside of the drawing furnace, in whichcase gas flow 4 is created inside the heating element 12 and theextension tube 15.

It is advantageous to manufacture the inner tubes 18 a, 18 b from amaterial, whose thermal expansion is as small as possible at thetemperature range used, it is especially preferable to use tubesmanufactured of graphite. The thermal expansion of graphite tubes, whoseheight is typically around 10 mm, is, in practice, non-existent. Forexample, a graphite heating element 12 expands in the drawing furnace 10on the temperature range used only about 5 mm, even though its totallength is around 500 mm. Because of the substantially non-existentthermal expansion of the inner tubes 18 a, 18 b the hole 18 c betweenthe inner tubes remains substantially the same over the entiretemperature-range, in which case arranging the gas flow 4 as desired canbe easily arranged.

The outer tube 17 a of the gas tube 16 is preferable to manufacture of athermally and mechanically strong material, such as steel. The change inthe size of the channel 17 b of the outer tube 17 a, because of thermalexpansion, is irrelevant in practice, if the channel is dimensionedlarge enough, in which case the gas flow 4 inside the drawing furnace 10can be kept as desired with the hole 18 c between inner tubes 18 a, 18b. It is preferable to form the outer tube 17 a from one uniform piece,but the outer tube can also be formed of more pieces, if necessary.

When using inner tubes 18 a, 18 b of the gas tube 16, which are made ofmaterial that substantially does not expand thermally, the holes 18 cformed in them remain substantially constant in size when thetemperature changes, if the pressing force affecting the inner tubesalso remains substantially constant. Thus the gas flow 4 through theholes 18 c also remains substantially constant. The vertical location ofthe holes in relation to the outer tube 17 a of the gas tube 16 changeswhen the length of the heating element 12 changes, and this change is tobe taken into account when forming the channel 17 b of the outer tube 17a of the gas tube, in order to keep gas flow 4 as desired in differentoperating modes. In the advantageous embodiment according to the exampleof FIGS. 1 and 2, the channel 17 b of the outer tube 17 a of the gastube 16 is formed to be so wide, that it is connected to the holebetween inner tubes 18 a, 18 b for the entire range of the length ofheating element 12.

A cover part 20 is arranged over the gas tube 16 in the upper part ofthe drawing furnace 10, which part closes the connection of the heatingelement 12 and the insulating layer 13 up to the outside space of thedrawing furnace. If necessary, it is possible to arrange cooling to thecover part 20 by using, for example, cooling channels 21 formed to thecover, in which channels a suitable medium is circled, such as coolingwater. Cooling the cover part 20 prevents too much heat transferring tothe other structures of the drawing furnace.

According to the invention, the cover part 20 is now fitted in relationto the gas tube 16 in such a manner, that the cover part can movevertically. It is advantageous to attach the cover part 20 to the gastube 16 with flexible fastening means 22, for example by springs, whichallow the cover part to move vertically. The change in the verticallength of the heating element 12 creates the vertical movement of thecover part 20, which change is created by the thermal expansion of theheating element 12 following the change of temperature. As a result ofthe change in the length of the heating element 12 the lower inner tube18 b arranged against the heating element moves vertically substantiallya range corresponding to the change in length. In which case the upperinner tube 18 a also moves vertically substantially the same rangecorresponding to the change in the length of the heating element 12.Thus the cover part 20 arranged above the gas tube 16, against the innertube 18 b, moves also vertically when the length of the heating element12 varies. The movement of cover part 20 is substantially the same asthe vertical length change of heating element 12 and the verticalmovement of the inner tubes 18 a, 18 b of the gas tube 16, when theinner tubes substantially are not thermally expanded.

According to the basic idea of the invention, the cover part 20 isarranged in such a manner that a substantially constant force isdirected at the inner tubes 18 a, 18 b between the cover part andheating element 12 in all the modes of operation of the drawing furnace10. Thus the level of sealing of the upper part of the drawing furnace10 remains constant in all modes of operation of the drawing furnace,and the level of sealing is also known. The substantially constant forcedirected at the inner tubes 18 a, 18 b between the cover part 20 and theheating element 12 is created with fastening means 22, which allow thecover part to move vertically. There can be one or more fastening means22, preferably from four to six. Preferably the fastening means 22 arearranged symmetrically in relation to the vertical central axis 10 x ofthe drawing furnace 10.

The fastening means 22 can be formed in several different ways. In anadvantageous embodiment, a flexible fastening means 22 is formed of anarbor and a spring. The arbor is attached through the cover part 20 tothe gas tube 16 and a spring is placed on top of the cover part 20around the arbor, and a sheet is placed at the end of the spring. Thesheet, which has a hole in the middle, is placed around the arbor insuch a manner, that the sheet cannot withdraw from around the arbor. Thelocking of the sheet can be done by, for example, a screw nut at the endof the arbor. By changing the location of the sheet, the pressing forceof the spring is affected and thus the pressing forces affecting theboundary surfaces of the inner tubes 18 a, 18 b. From the point of viewof the desired function of the sealing, it is essential that the flexingarea of the fastening means 22 covers at least the area, over which thelength of the heating element 12 may vary. If the play of the fasteningmeans 22 ends before the varying of the length of the heating element12, this results in the pressing forces of the sealing changing.

In another advantageous embodiment, the fastening means 22 is formed ofan arbor and a weight. First, the arbor is attached through the coverpart 20 to the gas tube 16 in such a manner, that the cover part canmove parallel to the arbor. After this, one or more weights are placedon top of the cover part 20 around the arbor. The total mass of coverpart 20 and the weight creates a constant pressing force to the innertubes 18 a, 18 b of the gas tube. It is possible for the length of theheating element to vary, and at the same time, to move the inner tubes18 a, 18 b of the gas tube 16 and the cover part 20 vertically, withoutthe pressing force directed at the sealing changing.

In one embodiment of the invention the pressing force of the cover part20 is created pneumatically, and it is clear, that the invention is notdependent on the manner the pressing force is created.

FIG. 2 presents, in principal, the drawing furnace 10 according to theembodiment of FIG. 1, and the mutual location of the different parts ofthe oven when the heating element 12 is hot and it is longer than in therest position shown in FIG. 1 because of thermal expansion. Restposition refers to a state when the heating element 12 of the drawingfurnace 12 is cold.

The length of the heating element 12 grows in relation to the length ofthe rest position when the temperature increases. When the verticallength of the heating element 12 has increased, also the inner tubes 18a, 18 b of the gas tube 16 have moved upwards substantially thecorresponding length. In the example, the cover part 20 of the drawingfurnace 10 is attached flexibly in relation to frame 11, in which casealso the cover part has moved upwards in relation to the frame part,substantially the range corresponding to the change in the length of theheating element 12. The sealing force between the cover part 20, theinner tubes 18 a, 18 b of the gas tube 16, and the heating element 12 isnow equivalent to when the heating element is in the rest position,because the fastening means 22 direct a constant force at the cover part20, irrespective of the vertical position of the cover part.

The pressing force, which affects the boundary surface between the coverpart 20 of the drawing furnace 10 and the upper inner tube 18 a, and theboundary surface between the upper inner tube and lower inner tube 18 b,and the boundary surface between the heating element 12 and the lowerinner tube remains substantially constant by the effect of the fasteningmeans 22 over the entire range of the length of the heating element 12.Thus, because of the fastening means 22, which allow the verticalmovement of the cover part 20, the sealing between the cover part andthe upper inner tube 18 a remains constant when the length of theheating element 12 changes, as well as the sealing between the heatingelement and the lower inner tube 18 b.

When the length of the heating element 12 of the drawing furnace 10varies, the distance between the outer tube 17 a of the gas tube 16 andthe cover part 20 also changes, which distance is substantially the sameas the distance between the lower inner tube 18 a and the insulatinglayer 13, which, for its part, is substantially the same as the changein the length of the heating element in comparison to the rest position.

The method of attaching the cover part 20 according to the invention isnot dependent on the channels 17 b of the gas tube 16 and the holes 18 cused in feeding gas, nor on the cooling channel 21 of the cover part,but the method of attaching according to the invention can be used byretaining its basic idea also in such cover structures, where there areno channels, or in which the purpose of use is something other than theone presented in the example.

By combining the modes and equipment structures presented in connectionwith the different embodiments of the invention presented above, it ispossible to provide various embodiments of the invention, which complywith the spirit of the invention. For example, the gas tube 20 presentedin the embodiment can be connected to the same structure with either thecover part 20 or the frame part 11 of the drawing furnace 10, in whichcase the cover part is attached directly to the frame part. Therefore,the above-presented examples must not be interpreted as restrictive tothe invention, but the embodiments of the invention can be freely variedwithin the scope of the inventive features presented in the claimshereinbelow.

1-10. (canceled)
 11. A drawing furnace for manufacturing optical fiberof a substantially vertical fiber preform, the drawing furnacecomprising: a heating element surrounding the fiber preform, for heatingthe fiber preform; an outside insulating layer of the heating element; aframe part of the drawing furnace in order to place said heating elementand said insulating layer in the drawing furnace; a cover part in orderto close the area between said heating element and said frame part toinsulate the insulating layer and/or the area surrounding the insulatinglayer from the gas flow surrounding the fiber preform; and a gas tubefitted between said cover part and said heating element in order to feedgas to the gas area surrounding the fiber preform; wherein the coverpart is further fitted mobile in relation to the frame when the lengthof the heating element changes in such a manner that a force, which issubstantially constant and seals the gas tube to the end of the heatingelement, is directed at said gas tube in all modes of operation of thedrawing furnace.
 12. The drawing furnace according to claim 11, whereinthe force between the cover part and the heating element can beadjusted.
 13. The drawing furnace according to the claim 11, wherein thedrawing furnace comprises fastening means for pressing the cover partsubstantially with a constant force towards the heating element.
 14. Thedrawing furnace according to the claim 12, wherein the drawing furnacecomprises fastening means for pressing the cover part substantially witha constant force towards the heating element.
 15. The drawing furnaceaccording to claim 13, wherein one or more springs or the like are usedas fastening means.
 16. The drawing furnace according to claim 14,wherein one or more springs or the like are used as fastening means. 17.The drawing furnace according to claim 13, wherein one or more means,whose pressing force is created with gravity, are used as fasteningmeans.
 18. The drawing furnace according to claim 11, wherein the gastube comprises one or more outer tubes and one or more inner tubesarranged inside the outer tube, which said outer tubes and inner tubesare arranged concentrically in relation to the fiber preform.
 19. Thedrawing furnace according to claim 11, wherein the gas flow to the gasspace surrounding the fiber preform is arranged through the channels andthe holes placed in the gas tube, the openings of the holes opening tosaid gas space symmetrically surround the fiber preform substantially ina horizontal plane.
 20. The drawing furnace according to the claim 11,wherein the gas flow to the gas space surrounding the fiber preform isarranged through the channels placed in the gas tube and onesubstantially uniform and horizontal hole, the opening of the holeopening to said gas space surrounds the fiber preform substantially overthe entire circle.
 21. The drawing furnace according to the claim 20,wherein the gas flow to the gas space surrounding the fiber preform isarranged through the channels placed in the gas tube and onesubstantially uniform and horizontal hole, the opening of the holeopening to said gas space surrounds the fiber preform substantially overthe entire circle.
 22. The drawing furnace according to claim 19,wherein the holes/hole form a gas tube between the first inner tube andthe second inner tube.
 23. The drawing furnace according to claim 20,wherein the holes/hole form a gas tube between the first inner tube andthe second inner tube.
 24. The drawing furnace according to claim 21,wherein the holes/hole form a gas tube between the first inner tube andthe second inner tube.
 25. The drawing furnace according to claim 22,wherein the dimensions of the holes/hole placed in the gas tube remainsubstantially unchanged in all modes of operation of the drawingfurnace.
 26. The drawing furnace according to claim 1, wherein theheating element is tubular.